An enzyme which hydrolyzes the nerve gas diisopropylphosphorofluoridate (DFP) is found in cophalopod nerves and is trivially called "squid-nerve-type DFPase". It appears restricted to cephalopod nerve and its occurrence coincides with isothionade, HOCH2CH2SO3-, the major anion (also exclusively) of cephalopod nerve. It is speculated that DFPase plays a role in the metabolic pathway from cysteine to isothionate. However, sulfinates and sulfonates are very weak inhibitors of DFPase (Ki equals 10 X Km) whereas the major hydrolysis product of DFPase action, diisopropyl phosphate, is a strong inhibitor (Ki equals Km). This suggests that a phosphate diester or triester may interact with DFPase, but we have examined cyclic AMP and cyclic inositol phosphate without significant results. Squid-nerve-type DFPase has extensively purified, including a final step of isoelectric focussing. The highest activity obtained is 44 micromoles DFP hydrolyzed per minute and milligram of protein. To examine further other interactions of DFP and related organophosphates, single cells of Electrophorus electricus are being voltage-clamped. When such cells are bathed in solutions of DFP and related compounds, there is little change in the functional properties of the conducting or synaptic membranes, even at unusually high conentrations. Results with Soman suggest that this compound may affect Na ion currents, but again at high concentrations.